1. Field of the Invention
The present invention relates to measuring the total energy flow rate of combustible gas and, more particularly, to a method of taking a flowing sample which is proportioned to the total energy flow rate.
2. History of the Prior Art
The dollar value of BTUs contained in natural gas, and other combustible gases, has increased the need to measure accurately the total energy flow rate of gas moving through pipe line systems, both at points near the point of use of the gas, and at points which may be remote from the point of use. The conventional methods for determining total energy flow rate at a point in a gas pipeline generally involve the simultaneous (or at least contemporaneous) measurement of several gas parameters which are then employed as inputs into calculations ultimately producing a value for energy flow. For example, one approach is to measure the pressure drop across an orifice plate in the line to obtain a starting point for calculation of flow rate, and to simultaneously measure the temperature and pressure of the flowing gas and its composition at the time (the latter being measured by a gas chromatograph). The composition, pressure and temperature measurements provide the data necessary for calculation of the density of the gas at the orifice plate. The calculated density and the before mentioned pressure drop across the orifice plate provide the data necessary for calculation of the volumetric flow rate. The gas composition measurement, taken together with the known heat of combustion values for various compounds and elements, enables one to calculate the heat of combustion per unit volume. Finally, the calculated heat of combustion per unit volume can be multiplied by the calculated volumetric flow rate to give a value for energy flow rate.
It can be seen that this approach and other similar conventional approaches which involve the making of multiple measurements of gas properties or parameters suffer from the apparent disadvantage that each measurement or type of measurement involves measurement errors. The errors of the multiple measurements accumulate and contribute an error in the final calculated value, which error may be quite sizable. In addition, each measurement made on the gas involves a measuring entity comprising some quality of equipment which must be maintained, and further involves periodic calibrating of that equipment to the desired or best possible accuracy. Furthermore, such approaches, to the extent that they involve hand calculations, also present opportunities for calculation errors.